Inhibition of activated protein C anticoagulant function enhances coagulation but does not interrupt downstream...

Hemophilic arthropathy results from recurrent joint bleeding driven by impaired thrombin generation and a synovial environment favoring bleeding and chronic inflammation. Activated protein C (APC), abundant in hemophilic joints, may exacerbate cartilage degradation through enhanced matrix metalloproteinase activity. APC inhibition has emerged as a potential therapeutic strategy that may improve hemostasis and possibly protect joint tissues. The aim of the present work was to evaluate whether systemic inhibition of APC anticoagulant activity provides superior protection of cartilage compared with standard factor (F) VIII prophylaxis in a murine model of hemophilic arthropathy. FVIII-deficient mice underwent two induced knee hemarthroses. Animals received either extended-half-life FVIII, an APC-inhibitory peptide, or saline for three weeks. Plasma APC activity was measured using a chromogenic assay. Magnetic resonance imaging (MRI) scan assessed joint bleeding, and histopathology was evaluated using Valentino and standardized microscopic arthritis scoring of histologic sections (SMASH) scoring systems, with blinded analysis of synovitis, vascularity, and cartilage integrity. Both FVIII and APC inhibitor treatments reduced MRI scan-detected intra-articular bleeding compared with untreated controls. Histology confirmed synovial hypertrophy in all injured knees. Total Valentino and SMASH synovitis scores did not differ significantly between groups. Neither FVIII replacement nor APC inhibition resulted in measurable joint protection compared with untreated FVIII-deficient controls. Systemic inhibition of APC anticoagulant activity rebalanced coagulation but did not yield superior joint preservation. Differences from prior antibody-based APC inhibition studies likely reflect inhibitor design, dosing strategies, and the limited impact of systemic inhibition on synovial APC activity. Further work should explore targeted APC modulation within the joint microenvironment.